Certain types of printing systems are adapted for printing images on large-scale substrates, such as for museum displays, billboards, sails, bus boards, and banners. Some of these systems use so-called drop on demand ink jet printing. In these systems, a carriage which holds a set of print heads scans across the width of the substrate while the print heads deposit ink as the substrate moves.
Solvent based inks are sometimes used in these systems in which an infrared dryer is used dry off the solvent after the ink is deposited onto the substrate. Systems using solvent based inks are able to print on flexible substrates such as PVC materials and reinforced vinyl. However, solvent based inks are typically considered to be unusable for printing on rigid substrates such as metals, glass, and plastics. Therefore, to print on rigid, as well as flexible substrates, radiation-curable inks such as UV-curable inks are often preferred. For these systems, the ink is deposited onto the substrate and then cured in a post-printing stage. For instance, after the deposition of the ink, the substrate moves to a curing station. The ink is then cured, for example, by exposing it to UV radiation. In other systems, the UV radiation source for curing is mounted directly on the same carriage that carries the set of print heads.
During the printing process, UV curable ink must be cured within a short time period after it has been deposited on the substrate, otherwise ink with positive dot gain may spread out and flow away, or ink with negative dot gain may ball up and roll away. UV radiation sources mounted on the carriage are capable of emitting radiation at high enough energies to cure the ink within such time frames. However, a significant amount of power must be supplied to the UV radiation source to enable it to emit these high energies. Typical UV radiation sources are quite inefficient since most of the emitted radiation is unusable. In fact, upwards of 95% of the emitted radiation is not used because the source emits radiation with wavelengths over a spectrum which is much wider than the usable spectrum. In addition, to ensure that the required amount of radiation is transmitted to the ink, the carriage must scan across the substrate at moderate speeds, even though the print heads are capable of depositing ink onto the substrate at much higher carriage speeds.
It is desirable, therefore, to set (i.e. pre-cure) the ink rather than fully cure it as the ink is deposited on the substrate so that the ink does not spread or ball up, even though it is still in a quasi-fluid state (i.e. the ink is not completely hardened). The energy required to set the ink is typically about 5% of the energy necessary to cure the ink. Such an arrangement requires less power, and, therefore, facilitates using smaller UV radiation sources. In addition, a lower energy output requirement would allow the carriage to operate at a higher speed. Hence, images can be printed at a higher rate, resulting in a higher throughput.
The present invention implements an apparatus for setting radiation curable ink deposited on a substrate. Specifically, in one aspect of the invention, the apparatus includes a series of ink jet print heads which deposit ink onto the substrate, and a radiation source mounted laterally adjacent to the series of ink jet print heads. The amount of energy provided by the radiation source is sufficient to cause the radiation curable ink to set. The set energy is typically about 5% of the energy required to cure the ink.
In certain embodiments of this aspect, the radiation source is a UV source and the ink is UV-curable. The radiation source can be a multiplicity of light emitting diodes (LED). The LEDs are lighter and smaller and require less power to operate. Because LEDs are capable of emitting radiation within a very narrow wavelength band, for example, 365 nm, they are very efficient. The LEDs can be pulse-width modulated such that the LEDs are capable of operating over a wider power range than traditional glow bulbs, such as mercury vapor lamps.
Embodiments of this aspect of the invention can also include one or more of the following features. The series of ink jet print heads traverses across the substrate from about 10 inch/sec to about 60 inch/sec, and the power emitted by the radiation source is about 50 W/inch. The system can include a radiation curing station which cures the ink after it has been set. The radiation curing station can be mounted on a carriage which carries the series of ink jet print head, or the UV curing station can be a stand alone unit which may or may not be attached to the printing system.
The series of print heads can include a first set of print heads for depositing black ink, a second set of print heads for depositing magenta colored ink, a third set of print heads for depositing yellow colored ink, and a fourth set of print heads for depositing cyan colored ink.
In some arrangements, the first, second, third, and fourth set of print heads are aligned linearly along either side of an axis that is substantially orthogonal to an axis of travel of the series of ink jet print heads. In other arrangements, the first, second, third, and fourth set of print heads are aligned linearly along an axis that is substantially parallel to an axis of travel of the series of ink jet print heads.
There can be a second radiation source, in which case the series of print heads are located between the two radiation sources.
Related aspects of the invention include a method to set radiation curable ink during a printing process. The method includes depositing the ink onto the substrate with a series of ink jet print heads. As a carriage holding the print heads traverses across the substrate, the method includes setting the ink with radiation emitted from a radiation source that is positioned laterally adjacent to the series of ink jet print heads.
Embodiments of this aspect may include setting the ink with UV radiation, for example, radiation with a wavelength of about 365 nm. The radiation source can emit with a power of about 50 W/inch. The method may also include curing the ink after the ink has been set. The depositing step may include depositing black ink from a first set of print heads, depositing magenta colored ink from a second set of print heads, depositing yellow colored ink from a third set of print heads, and depositing cyan colored ink from a fourth set of print heads. Still other aspects, features, and advantages follow.